One of the aims of green chemistry is to switch from organic solvents to water for organic transformations, to reduce environmental problems and contribute to safety. Palladium (Pd)-phosphine complexes are highly active and widely used in organic catalyses, with organic solvents, although Suzuki chemistry can be performed in water/EtOH systems. Water-soluble phosphine catalysts are available, but as many substrates are insoluble in water, they have been received less attention. However, if Pd-phosphine complexes, combined with a solid support, could undertake catalyses in water, it would be a powerful tool in organic synthesis.

Now, Y. Uozumi from the Institute for Molecular Science, Okazaki, Japan, reports the preparation of Pd-phosphine complexes bound to an amphiphilic polymer resin, and their success as catalysts for various reactions in aqueous media (J. Synth. Org. Chem.,Jpn., 2002, 60, (11), 1063–1068).

Polystyrene-poly(ethylene glycol) (PS-PEG) is a commonly used resin, functionalised for attachments. PS-PEG resin beads show relatively uniform swelling in solvents of different polarities. A PS-PEG resin with an amino group was chosen as a support for phosphines. The resin-supported phosphines were then treated with excess [pdCl(η³–C₃H₅)]₂ (Pd:P = 1.2:1) to form a Pd monophosphine complex.

The Pd-phosphine-PS-PEG resin was successfully tested for various transformations in water, such as allylic substitution, hydroxycarbonylation of aryl halides, the Heck reaction, cross-couplings of aryl halides and allyl acetates with arylboron reagents, asymmetric allylic substitution, etc. The catalyst could be recovered and reused without significant loss of activity and selectivity. A novel chiral P,N -ligand was prepared on PS-PEG, and with Pd, in water, gave π-allylic substitutions of cyclic and acyclic substrates of enantioselectivity up to 99% ee.